1. Field of the Invention
The present invention relates to a device and a method for the detection of a charging voltage of at least one rechargeable battery.
2. Description of the Related Art
Due to their manifold advantages mobile terminals in the electric and entertainment industries become increasingly important. These devices depend on a location-independent power supply and, for reasons relating to the expenses therefor and to the environment, it is recommendable to use rechargeable batteries (hereinafter called accumulator). At the same time, there is an increasing need for inexpensive and compact accumulator chargers, as the terminals, too, are constantly manufactured smaller in size and more inexpensively. On the other hand, the chargers have to be reliable and flexible enough to guarantee a long service life of the accumulators, and they have to be capable of charging a variable number of rechargeable batteries at the same time.
In accumulator technology the most different methods are employed for detecting the charging state of an accumulator, such as the evaluation of the temperature or voltage gradient of the chargeable accumulator. A very exact method exploits the effect that the accumulator voltage is reduced when the charging process is continued after a maximum was obtained. This method is also designated with ΔU detection. FIG. 1 schematically shows both the temperature and the voltage curves of a 1.2V accumulator cell during a charging process. As illustrated, the temperature of the accumulator continuously increases with the charging time. The cell voltage likewise continuously increases with the charging time, however, only up to a maximum value. Afterwards it decreases despite the continued charging process.
Once the charging voltage has reached its maximum, the accumulator has been charged up to its maximum capacity. Any additional charging beyond this point signifies that the accumulator is overcharged and, thus, damaged. Therefore, it is important that the charging process be stopped, if possible immediately after the voltage maximum has been exceeded. This requires that the negative voltage change, also called −ΔU, is exactly and reliably enough detected after the maximum value has been exceeded. With a cell voltage of 1.5V the value required therefor typically is 5 mV per cell or, expressed in percentage, typically 0.33% of the cell voltage.
In charging technologies the evaluation of the accumulator voltage and, thus, the detection of the −ΔU is typically performed with the aid of a microcontroller. The analog accumulator voltage is thereby supplied to an analog/digital converter (A/D converter) directly, or by means of a voltage divider, and is digitalized. For this purpose 8 bit or 10 bit A/D converters are commonly used.
For guaranteeing the necessary interference immunity when the negative voltage change −ΔU is measured by means of an A/D converter, only those signal changes are taken into account when the digital signals of the A/D converter are evaluated, that have a size of at least 2 LSB (Least Significant Bit). FIG. 6 shows, summarized in a table, the values of the maximum relative resolution of the voltage change −ΔU for both, an 8 bit A/D converter (8 bit ADC) and for a 10 bit A/D converter (10 bit ADC) with an evaluation exactness of 2 LSB. The values are issued for differently large digital output values of the A/D converters. The relative resolution with respect to smaller output values of the 8 bit A/D converter is clearly worse than the required typical 0.33%.
Relative to a definite number of accumulator cells the demand for accuracy in the −ΔU detection of at least 0.33% can, therefore, only be maintained by a 10 bit A/D converter, which is, however, clearly more expensive in comparison with an 8 bit A/D converter.
As the relative accuracy of larger voltages and, thus, in the upper range of the input voltage of the A/D converter is better, it should be made sure that the signal for the charging voltage of the accumulators or the accumulator package always ranges in the upper input voltage range of the A/D converter so as to achieve an as large as possible relative signal resolution.
If a variable number of accumulator cells is to be charged in one accumulator charger, e.g. 5 to 15 cells, a display of the voltage situation of the accumulator cells by a passive coupling, e.g. by a simple voltage divider, is no longer possible, if an accuracy in the −ΔU detection of 0.33% is to be achieved at the same time. FIG. 7 shows a table illustrating this fact by means of an example of a 10 bit A/D converter.
For obtaining even better resolutions in the −ΔU detection, microcontrollers and A/D converters having a higher resolution may be used.
Document U.S. Pat. No. 5,973,480 discloses that a microcomputer is used for setting a desired quotient of the accumulator cell voltage by additionally connecting resistors. A voltage division circuit divides the voltage range of the accumulator in correspondence with the measured range of the A/D converter downwardly. In view of a digital unit (1 LSB) of the 8 bit A/D converter this method results in an accuracy of 3.19 mV per accumulator cell.